Insulated gate bipolar transistors (IGBT) have been widely used, for example, as power switching devices in the application of motor PWM control inverters. The insulated gate bipolar transistor, which is of voltage driven type, can be easily handled in use, and has been thus required to have a large capacity to meet the needs of the market. In this situation, a module structure has been employed in which a plurality of IGBT chips are integrated within the same package.
In a MOS control type device, such as IGBT, an emitter electrode and a gate electrode are formed side by side on the main surface of a semiconductor chip. Where such IGBT are incorporated into a package, a collector electrode formed on the lower surface of the semiconductor chip is mounted on a support plate that is a metal base serving also as a heat radiator, and this support plate can be used as an external lead-through terminal for the collector electrode. The emitter electrode and gate electrode, on the other hand, need to be connected to corresponding external lead-through terminals through respective bonding wires. Since the bonding wire generally consists of an aluminum wire having a diameter of about 300 .mu.m, heat is scarcely dissipated from one side of the chip which bears the emitter electrode, and mostly dissipated from the other side of the chip which bears the collector electrode. Further, the internal wiring of this device has a large inductance.
To solve the above problems, a pressure contact type IGBT has been proposed that is mounted in a flat package having a ceramic insulating sleeve, similarly to a conventional thyristor or GTO thyristor. Since the IGBT is different from the GTO thyristor in that its main surface on which the emitter electrode is formed has a MOS structure, unlike the GTO thyristor, there arises a need to provide a collecting electrode in a region other than this MOS structure and apply the pressure to this region, so as to assure electric characteristics and long-term reliability of the transistor.
With regard to the pressure contact type IGBT, a plurality of semiconductor chips are secured by soldering to the same support plate (metal base) while being positioned by positioning guides. The collecting electrode of each semiconductor chip contacts under pressure with the upper common electrode through a corresponding contact terminal block. The positioning guide formed of fluorine resin, for example, is used for positioning this contact terminal block and the semiconductor chip.
In the above-described known structure using wire bonding, however, the current capacity of the element is considerably limited since heat is hardly dissipated from the emitter-electrode side of the transistor though the collector-electrode side of the transistor undergoes heat dissipation. In the element having a large current capacity, the number of bonding wires connected to the emitter electrode is increased, and reaches several hundreds particularly when a plurality of IGBT are accommodated in the same package to provide a module. In this case, the internal wiring has an increased inductance, which may cause a large surge upon a switching operation of the IGBT, and also cause a problem of reduced operating reliability.
In an attempt to solve the problems of heat dissipation, inductance of the internal wiring and others in the above-described known structure, it is proposed to incorporate the IGBT into a flat package, similarly to a flat semiconductor device such as a GTO thyristor, and draw the emitter electrode and collector electrode formed on the opposite major surfaces of each IGBT to respective common electrode plates exposed to the upper and lower surfaces of the flat package. In this structure wherein the emitter electrode of each semiconductor chip extends over an insulating layer covering the gate electrode, however, there may arise stresses in the MOS structure due to application of pressure thereto when the common electrode plate on the side of the flat package comes into contact under pressure with the emitter electrode. The thus produced semiconductor chip cannot be used in practice as it is.
In view of the above, a structure called a collecting electrode that serves as current path and heat radiator and does not have a MOS structure is provided on the emitter-electrode side of the IGBT. The contact terminal block that is accurately positioned by means of the positioning guide is brought into contact with this collecting electrode portion, so that the stress due to the pressure is not applied to the MOS structure where the emitter electrode is present. Thus, the semiconductor chip and contact terminal block are accurately positioned by the positioning guide without any shift therebetween in the lateral direction. In the known flat type IGBT wherein collector electrodes of a plurality of semiconductor chips are soldered under pressure to the same support plate, however, only one of the semiconductor chips cannot be replaced by another one even if the semiconductor chip is found defective after assembling of the chips with the flat package, thus causing a problem of considerably increased manufacturing cost of the semiconductor device. For example, a semiconductor device in which 12 semiconductor chips are combined together becomes defective if only one of these semiconductor chips becomes defective after assembling.